REGULATORY ACTIONS, Summary of Regulatory Actions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			12 Months Ended			12 Months Ended										12 Months Ended		12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011 NPC [Member]	Dec. 31, 2010 NPC [Member]	Dec. 31, 2011 NPC [Member] DEAA [Member]	Dec. 31, 2010 NPC [Member] DEAA [Member]	Dec. 31, 2009 NPC [Member] DEAA [Member]	Dec. 31, 2011 NPC [Member] REPR [Member]	Dec. 31, 2011 NPC [Member] TRED [Member]	Dec. 31, 2011 NPC [Member] EEPR Base [Member]	Dec. 31, 2011 NPC [Member] EEPR Amortization [Member]	Dec. 31, 2011 NPC [Member] EEIR Base [Member]	Dec. 31, 2011 NPC [Member] EEIR Amortization [Member]		Dec. 31, 2011 NPC [Member] GRC [Member]	Dec. 31, 2010 NPC [Member] GRC [Member]	Dec. 31, 2008 NPC [Member] GRC [Member]	Dec. 31, 2011 SPPC [Member]	Dec. 31, 2009 SPPC [Member] DEAA [Member]	Dec. 31, 2011 SPPC [Member] SPPC Gas [Member] DEAA [Member]	Dec. 31, 2010 SPPC [Member] SPPC Gas [Member] DEAA [Member]	Dec. 31, 2009 SPPC [Member] SPPC Gas [Member] DEAA [Member]	Dec. 31, 2011 SPPC [Member] SPPC Gas [Member] STPR [Member]	Dec. 31, 2010 SPPC [Member] SPPC Gas [Member] GRC [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member] DEAA [Member]	Dec. 31, 2010 SPPC [Member] SPPC Electric [Member] DEAA [Member]	Dec. 31, 2009 SPPC [Member] SPPC Electric [Member] DEAA [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member] REPR [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member] TRED [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member] EEPR Base [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member] EEPR Amortization [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member] EEIR Base [Member]	Dec. 31, 2011 SPPC [Member] SPPC Electric [Member] EEIR Amortization [Member]	Dec. 31, 2010 SPPC [Member] SPPC Electric [Member] GRC [Member]	Dec. 31, 2008 SPPC [Member] SPPC California [Member] GRC [Member]
Ely Energy Center [Abstract]
Spending amount approved by Public Utility Commission of Nevada on Ely Energy Center	$ 130
Amount spent on Ely Energy Center and recorded as other deferred asset	58.0
Farming Asset Reclassification		23.1
Development And Farming Asset Reclassification																	11.6
EEC Development Costs		23.2
Energy Efficiency Implementation Rate [Abstract]
Approximate revenues filed for estimated reduced kWh sales from energy efficiency program		35.1															7.6
Increases to EEIR base rates authorized		14.5															2.6
Pre-tax adjustment to earnings for revenue previously recorded		4.5															4.1
Revenues recognized for EEIR		15.5															2.5
Energy Efficiency Program Rate [Abstract]
Frequency of rate filings for energy efficiency program costs (in years)	3 years
Approximate costs filed for implementing energy efficiency programs		71.0															12.1
Increases to Energy Efficiency Program base rates authorized		58.4															9.7
EEPR revenues recorded		37.3															6.2
GRC Filings [Abstract]
Requested Increase In General Rates														249.9		305.7							4.3												8.9
Requested return on equity (in hundredths)		11.25%														11.00%							10.75%											10.75%	11.40%
Requested rate of return (in hundredths)		8.64%														8.88%							5.48%											8.14%	8.81%
Recovery, excluding AFUDC, for the expansion at the Harry Allen Generating Station		635.9
Increase in General Rates														158.6		222.7							2.7											13.1	5.5
Percentage Overall Increase In General Rates														8.30%		9.80%							1.93%											1.90%	8.00%
Approved ROE		10.00%														10.50%							10.00%											10.10%	10.70%
Allowed ROR		8.09%														8.53%							5.15%											7.86%
Dsm Costs		17.7
Mohave Closure Costs		12.7
EWAM Postponement Amount		46.9
Net Rate Case Adjustments														15.9
Revenue Requirement Subject To Change:
Anticipated Effective Date				Oct. 2011			Oct. 2011	Oct. 2011	Oct. 2011	Oct. 2011	Oct. 2011	Oct. 2011							Oct. 2011			Oct. 2011			Oct. 2011			Oct. 2011	Oct. 2011	Oct. 2011	Oct. 2011	Oct. 2011	Oct. 2011
Requested and Amended Revenue Requirement	(28.8)	(60.6)		(188.9)			8.6	18.1	58.4	21.3	17.1	4.8	[1]						(29.1)			0.3		(50.9)	(115.9)			38.0	9.1	9.7	4.6	3.1	0.5
Present Revenue Requirement	(16.7)	18.0		(101.0)			29.8	16.3	58.4		14.5								(16.7)					(42.7)	(99.5)			36.6	7.9	9.7		2.6
$ Change in Revenue Requirement	(12.1)	(78.6)		(87.9)			(21.2)	1.8		21.3	2.6	4.8							(12.4)			0.3		(8.2)	(16.4)			1.4	1.2		4.6	0.5	0.5
DEAA Filings [Abstract]
Requested rate increase (decrease)						72.1															8.7						25.9
Requested rate increase (decrease) percentage (in hundredths)						3.18%												2.69%			4.71%
Deferred costs refund requested			102.0			77.5														17.0	8.7					101.0	19.8
Amount Of Over Collection Against Deferred Btgr Debit Balance					102
Deferred Base Tariff General Rate balance															$ 95.8

[1] In accordance with Alternative Revenue Accounting, NPC recognized approximately $4.8 million in revenues pertaining to 2010. Based on the order from the PUCN in May 2011, which clarified the calculation of EEIR revenues, NPC does not expect to record further revenue from this rate request; however, NPC does expect to collect approximately $4.8 million from its customers